Granular polyethylene resins produced in polymerization reactors are often compounded with conventional additives, such as ultraviolet stabilizers, antioxidants, antiblock agents, slip agents, processing aids, and other additives well-known in the art. With or without addition of such additives, granular resins are frequently pelletized for ease in handling and transportation. Both of these processes, compounding and pelletization, can be carried out in mixers characterized by a feeding section where the granular resin is introduced to the mixer, and various zones in which the resin is fully or partially melted and mechanically mixed, typically by action of co- or counter-rotating, intermeshing or non-intermeshing twin screws. The resin is ultimately conveyed through a mixer discharge to form pellets, or directly conveyed to an extruder and further processed, such as by blowing or casting into a film.
Undesirable characteristics of polyethylene resins which are processed in such mixers include the presence of small, black particles (“black specks”) which appear in the pelletized resin and in articles subsequently formed of such resin, and imperfections generically termed “gels” which are apparent especially in films formed of such resins. The presence of black specks and gels makes the resin unusable for many commercial purposes, and such resins must be used as sub-prime or off-spec material. Problems of black specks and gels are generally more prevalent in resins processed in mixers, such as compounding/pelletizing mixers, and less prevalent in resins processed in extruders. However, it is often desirable to process polyethylene resins in a mixer rather than an extruder, due to the particular resin properties, the intended end use, and the availability of processing equipment in a commercial plant. In addition, increases in black specks and gels are frequently observed in resins after changes in the package of additives used, leading to time-consuming, costly, and often unsuccessful efforts to adjust resin, additive and processing parameters to eliminate such defects.
Documents considered to be relevant include U.S. Pat. No. 6,369,192; EP 0404552; U.S. Pat. No. 3,963,558; U.S. Pat. No. 3,799,234; U.S. Pat. No. 3,684,252; and U.S. Pat. No. 6,106,761.